How do you prove what a substance actually is? Analytical chemistry gives us a toolkit: checking purity from a melting point, separating mixtures by chromatography, and running quick colour tests for particular ions and gases. These are the “detective” skills of chemistry.
Purity and melting point
A pure substance melts sharply at one fixed temperature. An impurity lowers the melting point and makes it melt over a range of temperatures. So a sharp, correct melting point is good evidence of purity.
Chromatography and Rf
In paper chromatography a solvent carries the components of a mixture up the paper at different rates, separating them into spots. Each spot has a retention factor:
| Quantity | Meaning |
|---|---|
| Rf | distance moved by spot ÷ distance moved by solvent |
Rf is always between 0 and 1 and is the same for a given substance in a given solvent, so it helps identify components.
Flame tests, ion tests and gas tests
Flame tests identify some metal cations by the colour they give a flame:
| Ion | Flame colour |
|---|---|
| Lithium, Li⁺ | red |
| Sodium, Na⁺ | yellow |
| Potassium, K⁺ | lilac (purple) |
| Calcium, Ca²⁺ | orange-red |
| Copper(II), Cu²⁺ | blue-green |
Cation tests (add sodium hydroxide): Cu2+ gives a blue precipitate; Fe2+ a green precipitate; Fe3+ a red-brown precipitate; Ca2+ a white precipitate insoluble in excess; Zn2+ and Al3+ give white precipitates that redissolve in excess NaOH. Ammonium (NH₄+) releases ammonia gas on warming (turns damp red litmus blue).
Anion tests: a halide gives a precipitate with acidified silver nitrate — chloride white, bromide cream, iodide yellow. A sulfate gives a white precipitate with acidified barium chloride. A carbonate fizzes with dilute acid, releasing CO₂.
Gas tests: hydrogen → squeaky pop; oxygen → relights a glowing splint; carbon dioxide → limewater turns milky; ammonia → turns damp red litmus blue; chlorine → bleaches damp litmus paper.
Exam technique: “Describe a test and its result” needs both parts — the reagent/method and the observation (colour or precipitate). Memory hook for silver-nitrate halides: “white, cream, yellow” as you go down chloride, bromide, iodide.